1. Field of the Invention
The subject invention generally pertains to electronic power conversion circuits, and more specifically to high frequency, switched mode power electronic converter circuits.
2. Description of Related Art
Most soft switching or zero voltage switching (ZVS) converters require a brief dead time between operation of the switches to achieve ZVS. The amount of dead time required is dependent on the current magnitudes and component values so that the chosen dead time is often close to the amount of dead time needed for a range of currents but often the fixed dead time results in additional losses because the switch is turned on too soon when there is applied voltage to the switch or too late after the switch body diode or other parallel diode has been conducting for a time and dissipating power at a rate greater than the switch would dissipate if it were turned on at the best possible time. What is needed is a simple gate circuit that senses the mosfet drain voltage and enables the mosfet at the instant when the drain source voltage drops to zero.
In some cases, particularly at high line and/or light load, the energy available to drive a zero voltage transition is insufficient to drive the switch voltage to zero volts. In the energy insufficient case the switch is driven down to a minimum value and then the switch voltage rises up until the switch is turned on, generally after some fixed time delay. When the switch is turned on in the energy insufficient case the switching losses can be considerable. What is needed is a simple gate drive circuit that can turn on the main switch when the switch voltage is at its minimum value for the energy insufficient case.
An object of the subject invention is to provide a circuit mechanism that can accomplish optimal gate timing for switches that turn on at zero voltage.
Another object of the subject invention is to provide a circuit mechanism that can accomplish optimal gate timing for switches that are intended to turn on at zero voltage when there is insufficient energy available to drive the switch to zero volts.
Further objects and advantages of my invention will become apparent from a consideration of the drawings and ensuing description.
Optimal gate timing is provided for zero voltage switches by a simple circuit consisting of a small mosfet and two rectifiers. For the case where there is insufficient gate drive energy to drive the switch to zero volts a simple circuit is provided that turns the switch on at the minimum switch voltage.